Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Work function of vanadium dioxide thin films across the metal-insulator transition and the role of surface nonstoichiometry

Vanadium dioxide (VO(2)) undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature and there is great interest in exploiting this effect in novel electronic and photonic devices. We have measured the work function of vanadium dioxide thin films across the phase transition using variable temperature Kelvin force microscopy (KFM). The work function is estimated to be ～5.15 eV in the insulating phase and increases by ～0.15 eV across the MIT. We further show that the work function change upon the phase transition is highly sensitive to near-surface stoichiometry studied by X-ray photoelectron spectroscopy. This change in work function is distinct from bulk resistance-versus temperature trends commonly used to evaluate synthesis protocols for such vanadium oxide films and optimize stoichiometry. The results are pertinent to understanding fundamental electronic properties of vanadium oxide as well as charge injection phenomena in solid-state devices incorporating complex oxides containing multivalence cations.     

Anomalous Resistance Hysteresis in Oxide ReRAM: Oxygen Evolution and Reincorporation Revealed by In Situ TEM

The control and rational design of redox‐based memristive devices, which are highly attractive candidates for next‐generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky‐type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so‐called deep Reset.     

Fundamental device design considerations in the development of disruptive nanoelectronics

In the last quarter of a century silicon-based integrated circuits (ICs) have played a major role in the growth of the economy throughout the world. A number of new technologies, such as quantum computing, molecular computing, DNA molecules for computing, etc., are currently being explored to create a product to replace semiconductor transistor technology. We have examined all of the currently explored options and found that none of these options are suitable as silicon IC's replacements. In this paper we provide fundamental device criteria that must be satisfied for the successful operation of a manufacturable, not yet invented, device. The two fundamental limits are the removal of heat and reliability. The switching speed of any practical man-made computing device will be in the range of 10(-15) to 10(-3) s. Heisenberg's uncertainty principle and the computer architecture set the heat generation limit. The thermal conductivity of the materials used in the fabrication of a nanodimensional device sets the heat removal limit. In current electronic products, redundancy plays a significant part in improving the reliability of parts with macroscopic defects. In the future, microscopic and even nanoscopic defects will play a critical role in the reliability of disruptive nanoelectronics. The lattice vibrations will set the intrinsic reliability of future computing systems. The two critical limits discussed in this paper provide criteria for the selection of materials used in the fabrication of future devices. Our work shows that diamond contains the clue to providing computing devices that will surpass the performance of silicon-based nanoelectronics.     

Two‐ and Three‐Dimensional Alignment and Patterning of Carbon Nanotubes

Alignment or patterning of carbon nanotubes (CNTs) is particularly important for fabricating functional devices such as field emitters, nanophotonics, nanoelectronics, and ultrahydrophobic materials. This work briefly reviews recent progress on the synthesis of two‐dimensional CNT patterns, and then particularly concentrates on describing the pillar‐shaped fabrication and very interesting patterns of three‐dimensionally aligned CNTs formed by pyrolysis of iron(II ) phthalocyanine. The possible formation mechanism of the structures is discussed. The Figure shows the pillar‐shaped alignment of three‐dimensional CNTs.     

A Strategy to Design High‐Density Nanoscale Devices utilizing Vapor Deposition of Metal Halide Perovskite Materials

The demand for high memory density has increased due to increasing needs of information storage, such as big data processing and the Internet of Things. Organic–inorganic perovskite materials that show nonvolatile resistive switching memory properties have potential applications as the resistive switching layer for next‐generation memory devices, but, for practical applications, these materials should be utilized in high‐density data‐storage devices. Here, nanoscale memory devices are fabricated by sequential vapor deposition of organolead halide perovskite (OHP) CH₃NH₃PbI₃ layers on wafers perforated with 250 nm via‐holes. These devices have bipolar resistive switching properties, and show low‐voltage operation, fast switching speed (200 ns), good endurance, and data‐retention time >10⁵ s. Moreover, the use of sequential vapor deposition is extended to deposit CH₃NH₃PbI₃ as the memory element in a cross‐point array structure. This method to fabricate high‐density memory devices could be used for memory cells that occupy large areas, and to overcome the scaling limit of existing methods; it also presents a way to use OHPs to increase memory storage capacity.     

New Vanadium Oxide Nanostructures: Controlled Synthesis and Their Smart Electrical Switching Properties

Control over the different polymorphs of vanadium oxide that possess electrical switching properties is advancing rapidly as a result of the need to address energy‐efficiency issues; an example of which is the intelligent regulation of infrared light demonstrated by these polymorphs. Recent advances in the development of new vanadium oxide structures as well as their promising electrical switching properties are summarized here. Theoretical analysis and experimental results suggest that the presence of infinite vanadium ion chains in the crystal structure plays a decisive role in determining the electrical properties of vanadium oxides. The successful synthesis of new vanadium oxide materials and their nanostructures not only promotes a mechanistic understanding of the temperature‐driven electrical switching properties but also provides the right materials for constructing smart devices that can selectively filter out infrared light.     

Electrochemical Tantalum Oxide for Resistive Switching Memories

Redox‐based resistive switching memories (ReRAMs) are strongest candidates for the next‐generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta₂O₅‐based devices by anodizing. This method allows to grow high‐quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical‐oxide‐based devices demonstrate superior properties, i.e., endurance of at least 10⁶ pulse cycles and/or 10³I–V sweeps maintaining a good memory window with a low dispersion in R_OFF and R_ON values, nanosecond fast switching, and data retention of at least 10⁴ s. Multilevel programing capability is presented with both I–V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.     

A large-area nanoscale gold hemisphere pattern as a nanoelectrode array

Two-dimensional (2D) nanostructure patterns have extensive applications in photonic devices, nanoelectronics, electrochemical devices, biosensors, catalysts and high-density magnetic recording devices. It remains a challenge to develop low-cost, high-throughput, high-resolution techniques for the fabrication of large-area (wafer-scale) 2D nanostructure array patterns with controlled feature size, shape and pitch. The present work has demonstrated a low-cost, high-throughput, high-resolution approach for the fabrication of large-area, high-quality nanostructure array patterns by nanosphere lithography combined with electroplating. The gold hemisphere array pattern obtained is capable of functioning as a nanoelectrode array (NEA) in which the gold hemispheres act as individual electrodes that are separated with an insulating polypyrrole (PPY) film. Cyclic voltammetry measurement has shown a sigmoid-shaped voltammogram, which is characteristic of electrochemical characteristics of a nanoelectrode array. NEAs are expected to find extensive applications in fundamental electrochemistry studies and electrochemical devices.     

Engineering Transport in Manganites by Tuning Local Nonstoichiometry in Grain Boundaries

Interface‐dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In particular, it has been proven that strain‐induced defects in grain boundaries of manganites deeply impact their functional properties by boosting their oxygen mass transport while abating their electronic and magnetic order. In this work, the origin of these dramatic changes is correlated for the first time with strong modifications of the anionic and cationic composition in the vicinity of strained grain boundary regions. We are also able to alter the grain boundary composition by tuning the overall cationic content in the films, which represents a new and powerful tool, beyond the classical space charge layer effect, for engineering electronic and mass transport properties of metal oxide thin films useful for a collection of relevant solid‐state devices.     

Alternative Plasmonic Materials: Beyond Gold and Silver

Materials research plays a vital role in transforming breakthrough scientific ideas into next‐generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.     

Unveiling the Dynamical Assembly of Magnetic Nanocrystal Zig‐Zag Chains via In Situ TEM Imaging in Liquid

The controlled assembly of colloidal magnetic nanocrystals is key to many applications such as nanoelectronics, storage memory devices, and nanomedicine. Here, the motion and ordering of ferrimagnetic nanocubes in water via liquid‐cell transmission electron microscopy is directly imaged in situ. Through the experimental analysis, combined with molecular dynamics simulations and theoretical considerations, it is shown that the presence of highly competitive interactions leads to the formation of stable monomers and dimers, acting as nuclei, followed by a dynamic growth of zig‐zag chain‐like assemblies. It is demonstrated that such arrays can be explained by first, a maximization of short‐range electrostatic interactions, which at a later stage become surpassed by magnetic forces acting through the easy magnetic axes of the nanocubes, causing their tilted orientation within the arrays. Moreover, in the confined volume of liquid in the experiments, interactions of the nanocube surfaces with the cell membranes, when irradiated at relatively low electron dose, slow down the kinetics of their self‐assembly, facilitating the identification of different stages in the process. The study provides crucial insights for the formation of unconventional linear arrays made of ferrimagnetic nanocubes that are essential for their further exploitation in, for example, magnetic hyperthermia, magneto‐transport devices, and nanotheranostic tools.     

Interfacial Charge Engineering in Ferroelectric‐Controlled Mott Transistors

Heteroepitaxial coupling at complex oxide interfaces presents a powerful tool for engineering the charge degree of freedom in strongly correlated materials, which can be utilized to achieve tailored functionalities that are inaccessible in the bulk form. Here, the charge‐transfer effect between two strongly correlated oxides, Sm_0.5Nd_0.5NiO₃ (SNNO) and La_0.67Sr_0.33MnO₃ (LSMO), is exploited to realize a giant enhancement of the ferroelectric field effect in a prototype Mott field‐effect transistor. By switching the polarization field of a ferroelectric Pb(Zr,Ti)O₃ (PZT) gate, nonvolatile resistance modulation in the Mott transistors with single‐layer SNNO and bilayer SNNO/LSMO channels is induced. For the same channel thickness, the bilayer channels exhibit up to two orders of magnitude higher resistance‐switching ratio at 300 K, which is attributed to the intricate interplay between the charge screening at the PZT/SNNO interface and the charge transfer at the SNNO/LSMO interface. X‐ray absorption spectroscopy and X‐ray photoelectron spectroscopy studies of SNNO/LSMO heterostructures reveal about 0.1 electron per 2D unit cell transferred between the interfacial Mn and Ni layers, which is corroborated by first‐principles density functional theory calculations. The study points to an effective strategy to design functional complex oxide interfaces for developing high‐performance nanoelectronic and spintronic applications.     

Biodegradable Natural Pectin‐Based Flexible Multilevel Resistive Switching Memory for Transient Electronics

Transient electronics that can physically vanish in solution can offer opportunities to address the ecological challenges for dealing with the rapidly growing electronic waste. As one important component, it is desirable that memory devices combined with the transient feature can also be developed as secrecy information storage systems besides the above advantage. Resistive switching (RS) memory is one of the most promising technologies for next‐generation memory. Herein, the biocompatible pectin extracted from natural orange peel is introduced to fabricate RS memory devices (Ag/pectin/indium tin oxides (ITO)), which exhibit excellent RS characteristics, such as forming free characteristic, low operating voltages (≈1.1 V), fast switching speed (<70 ns), long retention time (>10⁴ s), and multilevel RS behaviors. The device performance is not degraded after 10⁴ bending cycles, which will be beneficial for flexible memory applications. Additionally, instead of using acid solution, the Ag/pectin/ITO memory device can be dissolved rapidly in deionized water within 10 min thanks to the good solubility arising from ionization of its carboxylic groups, which shows promising application for green electronics. The present biocompatible memory devices based on natural pectin suggest promising material candidates toward enabling high‐density secure information storage systems applications, flexible electronics, and green electronics.     

The Physics of Light Emission in Halide Perovskite Devices

Light emission is a critical property that must be maximized and controlled to reach the performance limits in optoelectronic devices such as photovoltaic solar cells and light‐emitting diodes. Halide perovskites are an exciting family of materials for these applications owing to uniquely promising attributes that favor strong luminescence in device structures. Herein, the current understanding of the physics of light emission in state‐of‐the‐art metal‐halide perovskite devices is presented. Photon generation and management, and how these can be further exploited in device structures, are discussed. Key processes involved in photoluminescence and electroluminescence in devices as well as recent efforts to reduce nonradiative losses in neat films and interfaces are discussed. Finally, pathways toward reaching device efficiency limits and how the unique properties of perovskites provide a tremendous opportunity to significantly disrupt both the power generation and lighting industries are outlined.     

Structural visualization of polarization fatigue in epitaxial ferroelectric oxide devices

Ferroelectric oxides, such as Pb(Zr,Ti)O(3), are useful for electronic and photonic devices because of their ability to retain two stable polarization states, which can form the basis for memory and logic circuitry. Requirements for long-term operation of practical devices such as non-volatile RAM (random access memory) include consistent polarization switching over many (more than 10(12)) cycles of the applied electric field, which represents a major challenge. As switching is largely controlled by the motion and pinning of domain walls, it is necessary to develop suitable tools that can directly probe the ferroelectric domain structures in operating devices-thin-film structures with electrical contacts. A recently developed synchrotron X-ray microdiffraction technique complements existing microscopic probes, and allows us to visualize directly the evolution of polarization domains in ferroelectric devices, through metal or oxide electrodes, and with submicrometre spatial resolution. The images reveal two regimes of fatigue, depending on the magnitude of the electric field pulses driving the device: a low-field regime in which fatigue can be reversed with higher electric field pulses, and a regime at very high electric fields in which there is a non-reversible crystallographic relaxation of the epitaxial ferroelectric film.     

Direct Evidence of Lithium Ion Migration in Resistive Switching of Lithium Cobalt Oxide Nanobatteries

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two‐terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Li_xCoO₂ layer. These observations are very well correlated with the observed insulator‐to‐metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling – much further than the present cycling life of usual lithium‐ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics.     

Ramp‐Reversal Memory and Phase‐Boundary Scarring in Transition Metal Oxides

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO₂ and NdNiO₃, which undergo a metal–insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp‐reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron‐glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving “scars” in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.     

Resistive Switching of Individual,Chemically Synthesized TiO2 Nanoparticles

Resistively switching devices are considered promising for next‐generation nonvolatile random‐access memories. Today, such memories are fabricated by means of “top–down approaches” applying thin films sandwiched between nanoscaled electrodes. In contrast, this work presents a “bottom–up approach” disclosing for the first time the resistive switching (RS) of individual TiO₂ nanoparticles (NPs). The NPs, which have sizes of 80 and 350 nm, respectively, are obtained by wet chemical synthesis and thermally treated under oxidizing or vacuum conditions for crystallization, respectively. These NPs are deposited on a Pt/Ir bottom electrode and individual NPs are electrically characterized by means of a nanomanipulator system in situ, in a scanning electron microscope. While amorphous NPs and calcined NPs reveal no switching hysteresis, a very interesting behavior is found for the vacuum‐annealed, crystalline TiO_2–x NPs. These NPs reveal forming‐free RS behavior, dominantly complementary switching (CS) and, to a small degree, bipolar switching (BS) characteristics. In contrast, similarly vacuum‐annealed TiO₂ thin films grown by atomic layer deposition show standard BS behavior under the same conditions. The interesting CS behavior of the TiO_2–x NPs is attributed to the formation of a core–shell‐like structure by re‐oxidation of the reduced NPs as a unique feature.     

Ferroelectric BiFeO3 as an Oxide Dye in Highly Tunable Mesoporous All‐Oxide Photovoltaic Heterojunctions

As potential photovoltaic materials, transition‐metal oxides such as BiFeO₃ (BFO) are capable of absorbing a substantial portion of solar light and incorporating ferroic orders into solar cells with enhanced performance. But the photovoltaic application of BFO has been hindered by low energy‐conversion efficiency due to poor carrier transport and collection. In this work, a new approach of utilizing BFO as a light‐absorbing sensitizer is developed to interface with charge‐transporting TiO₂ nanoparticles. This mesoporous all‐oxide architecture, similar to that of dye‐sensitized solar cells, can effectively facilitate the extraction of photocarriers. Under the standard AM1.5 (100 mW cm^?2) irradiation, the optimized cell shows an open‐circuit voltage of 0.67 V, which can be enhanced to 1.0 V by tailoring the bias history. A fill factor of 55% is achieved, which is much higher than those in previous reports on BFO‐based photovoltaic devices. The results provide here a new viable approach toward developing highly tunable and stable photovoltaic devices based on ferroelectric transition‐metal oxides.